1. Field of the Invention
The present invention relates to a method of manufacturing a liquid crystal display device (LCD), and more specifically, a method of manufacturing an LCD requiring only 4 mask processes while preventing undercutting of a semiconductor layer.
2. Description of the Background Art
Generally, a conventional liquid crystal display device includes a gate bus line 70 and a data bus line 60 arranged in a matrix array as shown in FIG. 1. The LCD further includes a TFT 40 functioning as a switching element driven by an electrical signal from the gate bus line 70 and the data bus line 60 which is disposed at an intersection portion of the gate bus line 70 and the data bus line 60 and a pixel electrode 190a connected to the TFT 40.
The TFT 40 includes a gate electrode 70a which is derived from the gate bus line 70, a source electrode 60a which is derived from the data bus line 60 and a drain electrode 60b which faces the source electrode 60a. The gate electrode 70a is covered by a gate insulating layer (not shown). A semiconductor layer 80 is disposed over the gate electrode 70a on the gate insulating layer. The source electrode 60a and the drain electrode 60b are disposed at each side of the semiconductor layer. The drain electrode 60b is connected to the pixel electrode 190a through a contact hole 45 which is formed in the passivation layer (not shown). At the end of the gate bus line 70 and the data bus line 60, a gate pad 170 and a data pad 160 are connected to an output of the driving IC.
The elements are formed by using mask processes involving photo lithography. Each mask process includes the steps of coating a photo resist on a thin layer, patterning the photo resist using a mask by exposing and developing the photo resist, etching the thin layer and removing the remaining photo resist which is remaining on the thin layer. As the number of mask processes increases, the yields for producing acceptable, non-defective LCDs decreases, because each mask process may cause some inferiority in mask alignment, over-etching, under-etching and pin holes.
Therefore, much research and attempts have been made to reduce the mask processes required for manufacturing of LCDs. As a result, the number of mask processes required for forming an LCD has been reduced from 8 mask processes to 6 mask processes, to 5 mask processes and to 4 mask processes.
The present invention relates to a 4 mask process method of manufacturing an LCD. The conventional method related to manufacturing an LCD is explained with reference to FIGS. 2a-2i.
On a transparent substrate 100, a first metal layer 150 including aluminum (Al), molybdenum (Mo) or chromium (Cr) is deposited. After a photo resist 600 is coated on the first metal layer 150, the photo resist 600 is exposed using a first mask 501 which has a patterning portion 500a and an opening portion 500b as shown in FIG. 2a. If the photo resist 600 is the positive type, then the portion 600b which is exposed to the UV light through the opening portion 500b of the first mask 501 is changed to be easily removed by the developer and the covered portion 600a which is not exposed to the UV light by the patterning portion 500a of the first mask 501 is hardened so as not to be removed by the developer.
The covered portion 600a of photo resist remains on the metal layer 150 and the exposed portion 600b is removed by the developer according to the patterning of the first mask 501. After developing, the photo resist is patterned as shown in FIG. 2b.
After the patterned (remaining) photo resist 600a is hardened by baking, the first metal layer 150 is etched by wet or dry etching method and is stripped by the patterned photo resist 600a. Then a gate pad 170, a gate bus line 70 and a gate electrode 70a are formed as shown in FIG. 2c.
A gate insulating layer 55 including SiN.sub.x, SiO.sub.x or BCB (BenzoCycloButene), an intrinsic amorphous silicon (a-Si) layer 80a, an n+impurity doped amorphous silicon layer 80b and a second metal layer 180 are deposited sequentially. After a photo resist 600 is coated on the second metal layer 180, the photo resist 600 is exposed by using a second mask 502 as shown in FIG. 2d.
Similar to the exposing step of the first mask 501, the photo resist 600 is exposed and developed by using the second mask 502. Then a pattern 600a of the photo resist is formed on the second metal layer 180 as shown in FIG. 2e.
After the patterned (remaining) photo resist 600a is hardened by baking, the second metal layer 180 and n+impurity doped a-Si layer 80b are etched by wet or dry etching method and strip the patterned photo resist 600a. Then, a data bus line 60, a source electrode 60a which is derived from the data bus line 60 and a drain electrode 60b which faces the source electrode 60a are formed. The doped a-Si layer 80b is formed to have the same shape as the source materials (the data line 60, the source electrode 60b and the drain electrode 60c) as shown in FIG. 2f. Furthermore, a data pad which is not shown in figures may be formed at the end of the data bus line 60.
On the substrate 100 having the source materials, a passivation layer 155 which includes an inorganic material such as SiN.sub.x or SiO.sub.x or an organic material such as a BCB is deposited and/or coated. After a photo resist 600 is coated on the passivation layer 155, the photo resist 600 is exposed by using a third mask 503 as shown in FIG. 2g.
Similar to the exposing step of the first mask 501 and the second mask step 502, the photo resist 600 is exposed and developed by using the third mask 503. Then a pattern 600a of the photo resist is formed on the passivation layer 155 as shown in FIG. 2h.
After the patterned (remaining) photo resist 600a is hardened by baking, the passivation layer 155, the a-Si layer 80a and the gate insulating layer 55 are simultaneously etched by using SF.sub.6 /O.sub.2 gas and is stripped by the patterned photo resist 600a. Then, a TFT is completed and the gate bus line 70 and the gate pad 170 are exposed as shown in FIG. 2i. At the same time, some portions of the drain electrode 60b are exposed through a contact hole 45. On the passivation layer 155, the a-Si layer 80a and the gate insulating layer 55 are patterned and the drain electrode 60b is exposed but not etched by the SF.sub.6 /O.sub.2 gas because the etching rate of the drain electrode 60b is much larger than the etching rate of the other layers.
An ITO (Indium Tin Oxide) layer 190 is deposited on the substrate 100 having the TFT. After a photo resist 600 is coated on the ITO layer 190, the photo resist 600 is exposed by using a fourth mask 504 as shown in FIG. 2j.
Similar to the exposing step of the third mask 503, the photo resist 600 is exposed and developed by using the fourth mask 504. Then a pattern 600a of the photo resist is formed on the ITO layer 190 as shown in FIG. 2k.
After the patterned (remaining) photo resist 600a is hardened by baking, the ITO layer 190 is etched and stripped by the patterned photo resist 600a. Then a pixel electrode 190a and a gate terminal 190b on the gate pad 170 are formed as shown in FIG. 2i. At the same time, a data terminal (not shown) is further formed on the data pad 160.
According to the conventional method as described above, the a-Si 80a is undercut when the passivation layer 155, the a-Si layer 80a and the gate insulating layer 55 are simultaneously etched in the third mask process. The main reason for the undercutting is that the etching rate of the a-Si layer 80a is greater than the etching rates of the passivation layer 155 and the gate insulating layer 55. The side surfaces 1, 2, 3 and 4 of the gate insulating layer 55, the a-Si layer 80a, the passivation layer 155 and the photo resist 600a are also etched by the SF.sub.6 /O.sub.2 etching gas. Here, the a-Si layer 80a is much more affected and etched than the other layers so that the a-Si layer 80a is over-etched which results in the undercut as shown in FIG. 3. According to the conventional method, the TFT which has the undercut a-Si layer 80a has inferior characteristics and therefore, the quality of the LCD is bad.